The present invention relates to a method and apparatus for detecting the air-fuel ratio of an internal combustion engine from the composition of its exhaust gases and feedback controlling in accordance with the resulting detection signal the air-fuel ratio of a mixture to be supplied to the engine at a desired value.
Air-fuel ratio controlling methods known in the art are of the simple integral control type which is responsive to the output of an air-fuel ratio sensor. More specifically, the known methods are such that when the output of the air-fuel ratio sensor is indicative of a "lean mixture" state, the then current air-fuel ratio correction amount is simply changed to the rich mixture side and that when the sensor output is indicative of a "rich mixture" state, the then current air-fuel ratio correction amount is simply changed to the lean side. Thus, if, during a transitional engine operation, the base air-fuel ratio varies faster in speed than the correction by the integral control, the correction cannot follow the variation adequately. Also, in the event that the air-fuel ratio sensor is inactive, due to the fact that the feedback control of the air-fuel ratio will not be effected properly and so on, it is impossible to ensure a satisfactory air-fuel ratio control with the resulting deterioration of the exhaust gas emissions.
In view of these circumstances, the inventors have proposed a method of controlling the air-fuel ratio of a digital engine comprising an integration step for integrating the output signal of an air-fuel ratio sensor, and a storage step by which a value corresponding to the integrated data produced by the varying step is stored as an engine condition correction data in a nonvolatile read/write memory in accordance with the engine condition existing at the time of the integration step, whereby the air-fuel ratio of the engine is controlled in accordance with one of the engine condition correction data stored in the non-volatile memory which corresponds to the current engine condition and the integrated data.
This air-fuel ratio controlling method employing a nonvolatile memory is also disadvantageous in that if the engine misfires, the air-fuel ratio sensor is prevented from sensing an accurate air-fuel ratio thus failing to accurately control the air-fuel ratio of a mixture at a desired air-fuel ratio. For instance, if one of the engine cylinders fails to fire, the air-fuel ratio of the mixture supplied to the engine will be enriched to about 12.5 to 14:1 A/F. An engine condition correction data to be calculated from this wrong integrated data and stored in the memory (i.e., a data to be obtained by learning) will of course be a wrong one and it is not desirable to allow such a wrong correction data to be stored in the nonvolatile memory and used for air-fuel ratio controlling purposes. For instance, in the case of a system in which the integral control is stopped and the air-fuel ratio is increased when for example overheating of a catalyst forming an exhaust gas purifying device is detected from the occurrence of a misfire or the like, the utilization of such wrong correction data is also disadvantageous from the standpoint of preventing overheating of the catalyst in that the air-fuel ratio is also decreased or the mixture is enriched under the effect of the wrong correction data even if the integral control is being stopped. In other words, this type of known air-fuel ratio controlling method has the possibility of resulting in an erroneous air-fuel ratio control if there exists any fault condition in the engine system. In this case, the fault conditions of the engine system include for example the previously mentioned abnormal temperature rise in the catalytic converter as well as the detection by the air-fuel ratio sensor of "a rich mixture" state over a very long period of time, the detection of "a lean mixture" state over a very long period of time, the detection of a misfiring by a known type of misfire sensor and the disconnection of the wire harness interconnecting the air-fuel ratio sensor and the air-fuel ratio control unit proper.